Field of the Invention
The present invention relates to a condensing sheet, a backlight unit, and liquid crystal display device using the same.
Discussion of the Related Art
In recent years, among the variety of flat panel display devices, liquid crystal display (LCD) devices have been most widely used because of its features of excellent image quality, light weight, slim design, and low power consumption.
A liquid crystal display device displays an image by adjusting light transmissivity of liquid crystals having dielectric anisotropy by using an electric field. To this end, the liquid crystal display device is provided with a liquid crystal panel having liquid crystal cells arranged in the form of a matrix, and a backlight unit to emit light to the liquid crystal panel.
As light sources of the backlight unit, a cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), and a light emitting diode (LED) are used. Backlight units may be classified into edge-type backlight units and direct-type backlight units according to the position of the light source. Edge-type backlight units are configured such that a light guide plate is disposed at the lower portion of the liquid crystal panel and a light source is disposed at one lateral surface of the liquid crystal panel. Direct-type backlight units are configured such that a diffusion plate is disposed at the lower portion of the liquid crystal panel and a light source is disposed at the lower portion of the liquid crystal panel.
As disclosed above, a backlight unit is provided with a plurality of optical sheets at the upper portion of a light guide plate or diffusion plate. The optical sheets include at least one diffusion sheet, and at least one condensing sheet.
The diffusion sheet is provided with a base film formed of polyethylene terephthalate (PET), acrylic resin layers formed on both sides of the base film and containing a light-diffusing agent such as bead. The diffusion sheet diffuses incident light to prevent dazzling due to partial concentration of light.
The condensing sheet may be a prism sheet provided with a base film formed of PET, and prisms formed in a pattern on the upper surface of the base film. The condensing sheet allows the incident light to leave the condensing sheet in a direction perpendicular to the liquid crystal panel, thereby increasing luminance per unit area.
For example, as shown in FIG. 1 a condensing sheet 5 implemented as a prism sheet has triangular cross sections in which a first inclined surface 5a and a second inclined surface 5b are alternately repeated. The light incident upon the condensing sheet is divided into perpendicular light L1, which is perpendicularly incident, and inclined light L2, which is incident in an inclined manner.
Specifically, the perpendicular light L1 perpendicularly incident upon the rear surface of the condensing sheet 5 undergoes total reflection on the first and second inclined surfaces 5a and 5b of a prism, and is then recycled. The inclined light L2 incident upon the first and second inclined surfaces 5a and 5b of a prism at a predetermined angle is refracted at the first inclined surface 5a or the second inclined surface 5b of the prism and perpendicularly collected at a liquid crystal panel. When the inclined light L2 is incident, some light L2-1 of the inclined light L2 travels along the first inclined surface 5a or second inclined surface 5b of the prism and leaves the prism without being refracted. This phenomenon is called side lobe.
In the case of the condensing sheet of the related art, side lobes degrade light condensing efficiency of the condensing sheet and lower the luminance of the front of a liquid crystal display device and contrast ratio at the viewing angle. Accordingly, the condensing sheet of the related art needs to be improved.